Massive freshwater river flows stemming from glacier-fed flooding at the end of the last ice age surged across eastern Washington to the Columbia River and out to the North Pacific Ocean, where they triggered climate changes throughout the northern hemisphere, new research published this week in Science Advances shows.
The findings, “The role of Northeast Pacific meltwater events in deglacial climate change”, provide new insight into the role the North Pacific Ocean plays in the planet’s climate, said Alan Mix, an oceanographer and paleoclimatologist in Oregon State University’s College of Earth, Ocean, and Atmospheric Sciences and one of the study’s authors.
“We look to the past to give us context for what might happen in the future,” Mix said. “We didn’t know before this research that the increase in freshwater flows was going to trigger widespread changes. It tells us this system is sensitive to these kinds of changes.”
The lead author of the study is Summer Praetorius, a research geologist at the U.S. Geological Survey who first started constructing records involved in the project as a doctoral student at Oregon State more than a decade ago.
Praetorius, Mix and OSU co-authors Maureen Walczak, Jennifer McKay and Jianghui Du collected data and analyzed records from across the Northeast Pacific region. They also examined an aggregate of data spanning several decades that was collected by scientists around the globe. Alan Condron, a modeling expert from Woods Hole Oceanographic Institute, also contributed to the analysis.
The researchers used computer modeling to project the rapid movement of floodwaters during deglaciation between 10,000 and 20,000 years ago, showing the flow from the Columbia River traveling along the coastline north to the Gulf of Alaska, across the Bering Strait and to Japan, as well as northward into the Bering Sea and the Arctic Ocean.
Freshwater is less dense than saltwater, sitting on top of the saltwater like a blanket and mixing down with the saltwater slowly, Mix said. The layers of water can change how heat moves around in the ocean, leading to less moderating of the climate.
The Columbia River runs along the dividing line between the subtropical region of the Pacific to the south and the subpolar region to the north. While some of the freshwater flowing out during the floods traveled south and dissipated, more of the water went north, where it flowed like a river along the coastline.
“This spread of floodwaters along the Alaskan coast was a big surprise,” said Condron. “The model showed that water from the Columbia River can impact most of the North Pacific and might even leak across the Arctic Ocean and into the Atlantic.”
Marine geologists from Oregon State then worked like crime scene investigators, tracing the impacts of the floodwaters through time using chemical “fingerprints” left in fossil shells that were alive during the flooding but sank and accumulated in muddy sentiments on the ocean floor along the floodwater’s path.
Mix led an expedition to collect sediment cores along the path of the floodwaters in 2004. The cores were then stored in OSU’s Marine and Geology Core Repository while the research was underway.
“The expedition yielded a treasure trove of mud from places nobody had thought to examine,” he said. “It has taken more than a decade of painstaking work sifting through the mud we retrieved looking for fossil shells that could help tell the story of the floodwater’s impact.”
Praetorius used the data from the shells and the modeling to show how the repeated flooding over 1,000 years cooled the ocean, which in turn impacted the climate across North America.
“Our findings suggest that freshwater flows into the North Pacific can have far-reaching impacts, changing ocean temperatures and steering winds and storm tracks in North America,” she said. “What happens in the North Pacific won’t stay in the North Pacific, but instead will cause changes far and wide.”
The warming underway today is opposite of what occurred at the end of the last ice age, Mix said, but understanding circulation patterns in the North Pacific gives researchers insight into what might happen as more warm water flows into the North Pacific as the planet warms.
“What we expect in the future is lower river flows and warmer water in the North Pacific – the opposite of what we saw at the end of the last ice age,” Mix said. “But the past is still informative, because it tells us how the circulation system of the North Pacific works.”
Columbia River megafloods occurred repeatedly during the last deglaciation, but the impacts of this fresh water on Pacific hydrography are largely unknown. To reconstruct changes in ocean circulation during this period, we used a numerical model to simulate the flow trajectory of Columbia River megafloods and compiled records of sea surface temperature, paleo-salinity, and deep-water radiocarbon from marine sediment cores in the Northeast Pacific. The North Pacific sea surface cooled and freshened during the early deglacial (19.0-16.5 ka) and Younger Dryas (12.9-11.7 ka) intervals, coincident with the appearance of subsurface water masses depleted in radiocarbon relative to the sea surface. We infer that Pacific meltwater fluxes contributed to net Northern Hemisphere cooling prior to North Atlantic Heinrich Events, and again during the Younger Dryas stadial. Abrupt warming in the Northeast Pacific similarly contributed to hemispheric warming during the Bølling and Holocene transitions. These findings underscore the importance of changes in North Pacific freshwater fluxes and circulation in deglacial climate events.